Page 38 - Primer on Enhanced Oil Recovery

Basic physical and chemical concepts 29

Table 3.1 Dynamic viscosity at 20 C in cP (equal to mPI).
Water (pure) 1
Seawater 1.1
Ethanol 1.1
Kerosene 1.6
Various Brents 5 7
West Texas Intermediate Around 5
0
Boscan (10 API) .50,000

In System International (SI) the unit if viscosity is Poiseuile (PI). In Centimeter-
Gramm-Second system (CGS system) the unit of viscosity is Poise (P).
Unfortunately a single unit is a bit too big for everyday use, so we instead use one
hundred of it centiPoise (cP). It is worth remembering that 1 P 5 0.1 PI. All the
below assumes that the viscosity of liquid is not dependant on the sped of flow
(which is not true in all cases) we name this class of liquids Newtonian fluids.
The lower the viscosity the more fluid will move through a pipe at the same pres-
sure. It is much easier and cheaper to pump a liquid with low viscosity (Table 3.1).
Viscosity of an oil is linked to the chemical composition, API, temperature, pres-
sure and amount of dissolved gas in oil. Dependence on chemical composition is
o
quite complex and cannot be presented in simple terms. As the gravity of oil ( API
value) goes down the viscosity increases very steeply. Dissolved gas reduces oil
viscosity. It means that if the pressure in the reservoir drops and passes the bubble
point and gas separate from the oil and the oil will become more viscous. Below
bubble point oil viscosity increases with the pressure reduction, above bubble point
pressure the oil viscosity increases with the pressure.
The temperature has dramatic effect on viscosity. Even simple liquids like water
will have big changes of viscosity as temperature rises. For example water dynamic
viscosity will be reduced by the factor of almost four as we increase temperature
from 20 to 100 C. It is not unknown for the crude to reduce viscosity two times for

every 5 10 C during increase in temperature. It means that increase of oil tempera-

ture by 50 degrees in many cases can reduce oil viscosity more than 100 times. High
temperature in a reservoir is good for low viscosity and easy liquids movement.
A phase is an amount of matter with uniform chemical content and consistent
molecules (atoms) structural arrangements. A phase has the set of well-defined prop-
erties. Knowing the phase one can know its properties. The opposite is also true.
The phase diagram is a drawing which links a matter properties with some ther-
modynamic parameters. It is probably easier to understand complex diagrams start-
ing with a simple one, known and chemically simple liquid, water. As an example
on Fig. 3.1 there is a simplified and slightly imprecise water phase diagram. There
are four water states solid, liquid, vapor and supercritical fluid. Lines between
the states represent the equilibrium values. In the triple point liquid, vapor and solid
(ice) exist in thermodynamic equilibrium. By definition at the sea level atmospheric
pressure this is happening at approximately 0 C or at around 273 K. Above the